Most machine tools that use laser beams utilize a positioning system having mechanical and optical elements to position the beam relative to the workpiece. While it is a simple matter to measure the position of these mechanical and optical elements, it has been difficult to measure the position of the focused beam directly. Since it is the position of the focused beam that affects the workpiece, it is the position of the focused beam, and not the position of the mechanical and optical elements, that is most important when determining the accuracy of the machine tool.
State of the art laser machine tools position the laser beam relative to the workpiece using motor drive systems and digital computer control. The laser beam is normally aligned to the mechanical and optical elements so that it is positioned in a nominally fixed position relative thereto. Practically, the alignment of the focused beam relative to the mechanical and optical elements is imperfect, and some shift in the relative position of the focused beam and the mechanical/optical elements occurs over time.
Since the focused beam does not stay in a fixed position relative to the mechanical and optical elements of the beam positioning system, it becomes beneficial to have the capability to determine the location of the focused beam directly in a three dimensional coordinate system. In this way, any shift of the relative position of the focused beam can be detected and measured. This information can then be used to determine the position error, and a determination can be made if the error falls within the allowed tolerance band for the particular operation being performed by the machine tool. If the position error is too large, the information can be used to alert the operator that the beam should be recalibrated or realigned relative to the mechanical/optical elements before conducting further operations with the machine tool.
Without some means to measure the focused position of the beam, its position can only be approximately located by visually observing the sparks created when it is positioned at the surface of a metal target.
Certain laser machine tools are equipped with cameras which can be used to visually observe the beam position. In these systems, usually limited to YAG lasers, the camera uses the same mechanical and optical elements as the laser so that any change affects the laser and camera equally. While these systems allow the system user to observe the beam position via the camera, they are not easily automated and are limited to lasers that operate at a frequency in or near the visible spectrum. Also, the position of the focused beam in the radial direction (i.e. along the z axis in an xyz coordinate scheme) cannot be determined accurately using such visual systems.